The present invention relates to a paper feeder for feeding a sheet of paper between its driving and driven rollers, which are kept in compressive contact with each other. The invention also relates to an image forming apparatus.
A commonly used paper feeder includes a driving roller and a driven roller which are kept in compressive contact with each other and feeds a sheet of paper between the two rollers. The driven roller is biased under a preset pressure against the driving roller by a spring or the like so that a preset nip pressure can be applied to a standard sheet of paper of a preset thickness between the two rollers.
In general, about 60-300 μm thick sheets of paper are used with image forming apparatus or the like. When the commonly used paper feeder feeds a sheet of paper different in thickness from the standard sheet, the radial force acting on the driven roller differs from that for the standard sheet, so that the rotational load on this roller differs from that for the standard sheet.
When the commonly used paper feeder feeds a sheet of paper thicker than the standard sheet, the nip pressure on the thicker sheet is higher, so that the rotational load on the driven roller is higher. As a result, the thicker sheet is fed at a lower speed. When this feeder feeds a sheet of paper thinner than the standard sheet, the nip pressure on the thinner sheet is lower, so that the rotational load on the driven roller is lower. As a result, the thinner sheet is fed at a higher speed, so that its leading end buckles.
The difference in the rotational load on the driven roller may cause the cylindrical surfaces of the driving and driven rollers to rub against each other. This damages the roller surfaces and sticks paper dust to them, thus causing a paper jam or another feed failure.
JP-2000-240638A discloses a feed roller including a sleeve and a shaft. The sleeve has an internal gear formed on its inside. An external gear is fixed to the shaft. When no radial force acts on the sleeve, the driving force of the shaft is not transmitted to the sleeve. When radial force acts on the sleeve, the two gears mesh together to transmit the driving force of the shaft to the sleeve. If high tension is exerted on a sheet of paper being fed by the feed roller, radial force acts on the sleeve, so that the driving force of the shaft is transmitted to the sleeve. This prevents the sheet from being broken.
The rotational load on the feed roller can be switched between only two levels either to transmit the driving force of the shaft or not to transmit it to the sleeve. Accordingly, it is impossible to adjust the rotational load for various types of paper different in thickness. As a result, it is impossible for the feed roller to smoothly feed some types of paper different in thickness.
A driver is necessary for changing the rotational load on the feed roller. The feed roller is complex in structure, so that its manufacturing cost is high.